Control and servo systems, such as hydraulic or pneumatic systems, are well known and operate on the simple principle of transferring force from an applied location to an output location by means of a fluid. In hydraulic systems, the transfer is typically accomplished by means of an actuator cylinder having a piston contained therein pushing a substantially incompressible fluid through a fluid line to another cylinder, also having a piston, at a different location. One tremendous advantage to transferring force through a hydraulic system is that the fluid line connecting the two cylinders can be any length and shape, and can wind or bend through all sorts of positions separating the two pistons. The fluid line can also split into multiple other fluid lines thus allowing a master piston to drive multiple slave pistons. Another advantage of hydraulic systems is that it is very easy to increase or decrease the applied force at the output location. This hydraulic force multiplication is accomplished by changing the size of one piston relative to the other.
In most hydraulic systems, cylinders and pistons are connected through valves to a pump supplying high-pressure hydraulic fluid functioning as the substantially incompressible fluid. Spool valves are the most commonly used valves in hydraulic systems and can apply pressure to either the front or back faces of the piston inside the hydraulic actuators. When one side the actuator cylinder is pressurized, the spool valve simultaneously opens a return line to the opposite side of the actuator, allowing the substantially incompressible hydraulic fluid on the opposing side of the piston to bleed back into a return reservoir. This relieves any internal pressure that would oppose the movement of the actuator, and limits the work required by the actuator to only that which is needed to drive the external load. As a result, spool valves are ideally suited to hydraulic systems because they allow efficient control of the flow rate to achieve hydraulic force.
Still, in spite of the advantages of spool valves in hydraulic systems, existing spool valves have certain design limitations. Traditional spool valves have been designed to be actuated by either mechanical levers, electrical servos or internal control pressures called pilot pressures, which are provided by way of a pilot valve. Spool valves are commonly mounted in a cylindrical sleeve or valve housing with fluid ports extending through the housing, which can be opened or closed for fluid communication with each other by positioning the lands and recesses of the spool in appropriate locations within the sleeve. The working pressure is varied by displacing the valve spool to open or close the valve allowing varying amounts of pressurized fluid to flow from the supply reservoir.
In the case of electrical actuation, the valve is controlled by an electrical input current from an electrical source. The current may be related to the pressure in the system in that the greater the current supplied, the wider the pressure or supply port is opened allowing pressurized fluid to flow into and through the valve with less restriction. When the load pressure in the actuator finally equals the supply pressure then flow stops. In other words, a given current controls the size of the openings in the pressure or return ports, which in turn controls the flow rate of fluid into or away from the hydraulic actuator. In order for the system to operate correctly, there must a be constant pressure differential across the spool valve. Otherwise, as the load pressure approaches the supply reservoir pressure the valve loses linear response and its operation becomes unstable. Consequently, spool valves are typically operated in systems where the pressure of the source (i.e. the supply reservoir) is very high compared to the range of opposing load pressures, and the flow versus input current at a given pressure is linear in the usable region.
What this means is that the system, and particularly the load, is always in a pressurized state and cannot be freely moved by an external force or under its own weight. As such, the load cannot easily be moved without actual active actuation. In other words, the actuator cannot be passively back driven. Such a configuration is extremely inefficient, as active input in the form of pressurized fluid is required to displace or actuate a load, even in the presence of gravity or in response to momentum (e.g. while braking). The use of pressurized fluid creates a significant energy loss as new pressurized fluid must always be supplied in order to move and/or brake the load.
In addition to the current flow problems of traditional spool valves, classical hydraulic systems are problematic for several other reasons. First, complex controllers are needed to control the cycle times of valves and pistons. Second, cycle times for moving pistons are often long because large amounts of fluid are required to move output pistons. Third, the large quantity of fluid needed to drive output pistons requires constant pressurization of large reservoirs of fluid accumulators. Consequently, hydraulic machines typically require large amounts of hydraulic fluid for operation and therefore require large external reservoirs to hold the difference in the volume of fluid displaced by the two sides of any cylinder.
Classical spool valve devices are also limited in application because when a controlled flow is induced through a valve it generally translates directly into a controlled velocity of the actuator's piston. Consequently, complex system feedback devices must be used to convert the hydraulic energy from velocity inputs into a system based on load position. Introducing feed back control devices into the system limits its response to the bandwidth of the feedback loop and the responsiveness of the valves such that the time delays between the feedback devices and the valves make the system unstable when a resistive force is applied.
Still other problems exist with classical servo valves operating in classical servo systems. Due to the problems discussed above, these valves and systems are incapable of performing at high bandwidths without going unstable. In addition, significant amounts of energy may be lost due to leakage when not all of the valves in a multiple valve system are being used. Finally, the configuration of the spool can be limiting, with multiple lands and recesses formed in a single spool, and with the single spool functioning to open and close the pressure and return ports formed in the valve body.